Um. Accidently fermentation? wtf?

Ok, so I'm making a cyser, but that isn't the focus of this post.

I had about half a gallon left of my apple juice, and due to my fridge being broken (damn this apartment complex!) I immediately closed the apple juice bottle and placed it on my counter. 3 days later the apple juice container is sort of inflated (its plastic) I opened it up and relieved pressure. Just now I saw that it was once again inflated, and once again opened it and it relieved pressure. There are even bubbles rising in it, as if fermentation was taking place.

So my question is...is there fermentation taking place? Is this an example of open air fermentation? Is this safe to drink?

thank you for your responses in advance.

It is a fermentation of some sort. It might not be good though.

ok, I'm going to throw it away. thank you.

Well, might as well ride it out. See what happens.

Yeah you've most likely either got hard cider or apple cider vinegar